Embryos of the African clawed frog, Xenopus laevis, represent a long-standing model of vertebrate development. They are also used in FETAX (Frog Embryos Teratogenesis Assay-Xenopus) and similar assays of the developmental toxicity of chemicals and environmental samples. 2,3,7,8 tetrachlorodibenzo-p-dioxin (TCDD) is a potent developmental toxicant in most vertebrates. However, frogs are generally insensitive to TCDD toxicity, especially at early life stages. Thus, FETAX and other frog embryo toxicity tests may be poorly suited for determining the developmental toxicity of samples containing dioxin-like compounds. This research seeks to delineate the molecular mechanisms underlying TCDD insensitivity in frogs, using X. laevis as a model system. During the previous grant period, we cloned cDNAs encoding two distinct X. laevis aryl hydrocarbon receptors, AHR1A and AHR1B both of which exhibit extraordinarily low affinity for TCDD in vitro and unusually late onset of detectable signaling activity during development. The proposed project will elucidate the functions of the two AHRs and the regulation of their activity in embryo and tadpole stages. Specifically, we will (1) Dissect the transcriptional properties of AHR1A and AHR1B in reporter gene assays using transiently transfected cells; (2) Determine the timing of onset of AHR protein expression and functional AHR signaling during development; (3) Determine the role of AHR Repressor protein in the attenuation of AHR signaling in early embryos, and (4) Compare TCDD-induced lethality and elimination rates in tadpoles and early embryos. Understanding the molecular mechanisms of TCDD insensitivity in different life stages is important for determining the human health relevance of frog embryo toxicity assays. Moreover, the unique features of frog AHR function and expression may provide a novel perspective on the relationship between AHR expression, AHR activity, mechanisms developmental TCDD toxicity, and the endogenous functions of AHRs during vertebrate development.